For more than half a century, pressure-sensitive adhesive (PSA) tapes have been used as packaging means for holding, sealing, protecting and as masking tapes in painting operations. The use of radiant energy to improve the physical properties of PSAs and, particularly, of PSA coatings, can be found as early as 1960 in U.S. Pat. No. 2,956,904 (Hendricks) wherein the use of high energy electron beam radiation is disclosed to increase the cohesive strength of the PSA coating.
U.S. Pat. No. 4,181,752 (Martens et al) teaches the in situ polymerization of acrylic monomer and comonomer syrups subjected to ultraviolet radiation at a very precise intensity of exposure of 0.1 to 7 milliwatts per cm.sup.2 which optimizes the molecular weight of the resulting polymer. The photoinitiator selection is not as critical as the rate of ultraviolet dosage. Crosslinking of the adhesive by use of photoactive crosslinkers is disclosed; various benzophenone-type compounds are among the photoactive crosslinkers that are disclosed.
U.S. Pat. No. 4,165,260 (Stueben et.al.) discloses compositions comprising a poly(vinyl alkyl ether), a liquid monoacrylate monomer and a photoinitiator which compositions cure upon exposure to radiation to form a pressure-sensitive adhesive. Benzophenone is disclosed as the preferred photoinitiator.
Copolymerizable aromatic ketone monomers have been incorporated into pressure-sensitive adhesive copolymers. U.S. Pat. No. 3,998,997 discloses the use of copolymerizable hydroxy-aromatic ketone monomers at a level of at least 1% or an aromatic ketone monomer free of aromatic hydroxyl groups, e.g., diacetophenone acrylamides, at a level of two percent by weight, to improve the shear strength of an acrylate-functional pressure-sensitive adhesive. The patent discloses the use of multi-ethylenically unsaturated compounds as crosslinking agents, but does not address the compliance of the adhesive and does not disclose the use of ultra-violet radiation in preparing the disclosed adhesive.
A review article entitled "Radiation Curing of Pressure-Sensitive Adhesives", authored by Toshio Okada, published in the journal (Japanese language publication identified by Exhibit A of Applicants Information Disclosure Statement filed herewith) Vol. 20, No. 611984, discloses that copolymers of benzoin acrylate and an acrylate monomer such as 2-ethylhexylacryalte can be cured upon exposure to ultraviolet radiation to yield a pressure-sensitive adhesive having solvent-resistance, environmental resistance and heat resistance (above 200.degree. C.). It is believed that the degree of crosslinking of an adhesive having such properties would result in an adhesive having a very low degree of compliance.
The difficulty of adhering tape or other devices to the human skin has long been recognized. The irregular and complex surface of the skin presents obstacles in itself and the wide variation in the skin surface from individual to individual and from site to site on the same individual compound these obstacles. Acrylic PSAs have been used for many years in medical and surgical applications. An outstanding acrylic copolymer, of the type described in U.S. Pat. No. 2,884,126/RE 24,906 (Ulrich) has been employed in a porous, surgical adhesive tape, U.S. Pat. No. 3,121,021 (Copeland) with very acceptable skin adhesion performance. The advantages of an acrylic PSA in medical applications, e.g., less irritation to the skin, as well as the disadvantages which result from adhesion buildup (compliance failure) when the acrylic PSA is in contact with the skin for long periods of time were recognized in U.S. Pat. No. 3,321,451 (Gander). According to the aforementioned Gander patent, the irritation caused by removal of the tape was overcome by including in the acrylate adhesive polymer certain amine salts which made it possible to remove the tape by washing with water, a procedure which is not always feasible where high standards of sterility must be maintained.
In U.S. Pat. No. 3,475,363 (Gander), the inventor has attempted to overcome the objectionable compliance failure in acrylate PSAs by employing as a crosslinking agent dimethylaminoethyl methacrylate to ensure adhesion to the skin without deleterious effects. U.S. Pat. No. 3,532,652 (Zang), recognizes that acrylate PSAs are affected when used on surfaces which promote migration of oils and the like to the adhesive, thereby weakening their cohesive strength. Zang overcomes this problem by partially crosslinking his acrylate interpolymer with polyisocyanate. In U.S. Pat. No. 4,140,115 (Schonfeld), the inventor reduces the stripping of tissue cells when his acrylate PSA is removed from the skin by blending the PSA with an unreacted polyol having a fatty acid ester pendant moiety. This adhesive has a tendency to leave objectionable residue.
Another approach to modifying PSA tape for the purpose of controlling the degree of elastic compliance and viscous flow can be found in U.S. Pat. No. 4,260,659 (Gobran), which teaches how a plurality of superimposed adhesive layers having different gradients of shear creep compliance can meet the requirement of releasable adhesion to a plastic surface such as polyethylene. In U.S. Pat. No. 4,374,883 (Winslow), the shear creep compliance is achieved and the cohesive strength maintained by combining two layers of adhesive which reinforce each other. While both of these address the problem of modifying a PSA to improve compliance, neither deals with the kind of precision and control required in medical products which are adhered to the skin.